A PIP (picture-in-picture) TV is indicative of a television which is capable of displaying a stationary or moving sub-picture within the main picture as a part of it, which is rendered possible with the digitalization of the processing of image signals.
In such a PIP TV, first a picture can be displayed as the main picture through a tuner, and then, a moving or stationary picture from non-broadcast video sources such as VTR can be displayed as sub-pictures. Subsequently, a technique for swapping the sources of the main and sub-pictures was developed, and also, a system capable of simultaneously receiving two off-air broadcast video sources, namely a 2-tuner system, was developed.
Recently, the digitalization of the processing of video signals has been promoted more in order to process and display diversified video signals from the video signal origins such as public broadcasts, VTR, CATV, Video disc players or video cameras.
The digital tuner which has appeared in conformity with such trends is capable of receiving simultaneous video signals from a plurality of broadcasting sources, and is capable of outputting them after multiplexing them, so that a plurality of broadcasting can be simultaneously displayed on the TV screen. This technique, when applied to the PIP TV is called a multiple PIP(M-PIP).
Such a multiple PIP TV can display a plurality of sub-pictures within a main picture, and therefore, can show various new functions which are not seen in the more conventional PIP TV. One of the new functions is the multichannel scanning function wherein the digital signals from the digital tuner can be successively outputted, so that programs broadcast by different broadcasting stations or programs outputted from different video signal sources can be scanned, thereby performing the multichannel scanning function.
The conventional multichannel scanning method of such a multiple PIP TV was adopted, for example, by Matsushita Electric Company of Japan in its Hi-Fi VTR "NV-D21" sold on Dec. 1, 1986 (refer to the Journal Video Saloon, Dec. 1986, P 139).
In this method, first, the user manipulates the ten keys to input the channel to be scanned and to store the data into the system, and then, the user presses the channel scan key to set a plurality of sub-pictures on the screen. At the same time, the system successively writes into and reads the video signals for the selected and memorized channel from the relevant memory, so that the stationary images for the memorized channels are outputted to the respective sub-pictures, thereby performing the multi-channel scanning.
When selecting channels (when selecting screen multiplex broadcasting channels so as to assure a plurality of sub-pictures within a main picture) in the conventional multichannel scanning method, each of the channels can not be assigned to an independent multi-voice mode, but the sound signals are outputted based on the sound mode selected for the main picture.
That is, in the case where a sub-picture is displayed like a main picture, the sound signal will be outputted based on the sound mode selected for the basic main picture, with the result that the sound mode is fixed to be unvariable even in the case of a multichannel selection.
Accordingly, the diversification of the picture is attainable through a multichannel, but the diversification of sound modes can not be expected.